Microfluidics, the manipulation of microliter volumes in a channel with sub-millimeter dimensions, allow reduction in the required sample volume, a potential decrease in assay completion time and cost reduction for chemical reactions. Microfluidics manipulate fluids by using sequential and parallel analytical processes thus minimizing user manipulation of fluids.
Molecular arrays have been successfully used to perform analytical assays. Such arrays can be used for detection of antibody recognition, analysis of nucleic acid molecules, peptide detection, drug screening, genetic typing and fingerprinting, and disease diagnosis and other analyses known to those skilled in the art of molecular arrays.
An assay in an array can contain binding molecules of several disparate species of a single type or class of molecule (e.g., nucleic acid, or protein), each species being placed on one or more points, or features, on an array. Analytes, such as those found in blood or other body fluid, are usually washed over the entire array in a liquid medium. Analytes bind to specific features in the array because of specific interactions between the analytes and binding molecules contained either in the fluid and/or on the surface of the array. Examples of such specific interactions include, but are not limited to, ligand-receptor interactions, such as antibody-antigen interactions, nucleic acid hybridization, enzyme/substrate, and binding protein-nucleic acid interactions.
Ligand-receptor interactions comprise a number of steps which accomplish functions: (1) combining a test fluid with a specific binding reagent that makes the material of interest detectable (tagging), and (2) selectively retaining the analyte of interest in specific zones by using direct or indirect capture in those zones. These functions can further include: (1) introducing a fluid into a device, (2) tagging any analyte present by reacting the fluid with a specific binding reagent, (3) reacting the fluid containing any tagged analyte with an analyte capturing reagent, (4) removing the analyte of interest from the bulk of un-reacted fluid, (5) washing the region of the device containing the analyte capture reagent, and (6) detecting and analyzing the presence of the analyte in the region containing the analyte capture reagent.
One class of devices for conducting such receptor-ligand assays uses membranous material. In these devices, a membrane is used as a carrier for the specific binding reagent and the analyte capture reagents. Each is localized to specific zones on the membrane and movement of a fluid introduced to the membrane through these zones is accomplished by capillary migration. An example of this type of assay device is a “strip assay.”
Another class of devices for conducting ligand-receptor assays is based on “open format” assays. These assays involve the use of tubes, channels, or wells instead of membranes. Specific binding reagents and analyte capture reagents are immobilized on separate zones of the walls of the tubes, channels, or wells, and the reagents react with the fluid as the fluid flows through the zones. In these open format devices, various methods of fluid mechanics, e.g. pumps or gravity, can be used to induce movement of the fluid through the tubes, channels, or wells.
Another method of fluid mechanics used to induce fluid flow utilizes capillary forces in a microfluidic environment. The desire and challenge of combining a simple miniaturized device with processing a small sample volume includes the problem of accomplishing all the desired steps for chemical analysis while avoiding the use of pumps. It is therefore desirable to have a method and system that enables an assay device with channels capable of moving fluids by capillary flow and able to handle volumes necessary for reaction and detection associated with such assays.